A fluid flow sensor is generally a device for sensing a rate of fluid flow. There are various kinds of fluid flow sensors, including some that have a rotor pushed by a fluid driving a rotary potentiometer, or similar devices. Other flow sensors are based on sensors which measure a heat transfer caused by a moving medium, which is common for fluid micro-sensors. Velocimeters measure a velocity of fluids flowing through them. Hall effect sensors may also be used, for example, on a flapper valve to sense a position of the valve as displaced by a fluid flow.
Air flow around an aircraft is generally measured via a pitot tube. A pitot tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube is generally used to measure a local velocity at a given point in a flow stream and not an average velocity in a pipe or conduit. A basic pitot tube may consist of a tube pointing directly into the fluid flow that is open at one end and closed at another end. When the tube contains a fluid from the flow stream, a pressure can be measured; where a moving fluid is brought to rest (stagnates) as there is no outlet to allow flow to continue. The pressure is a stagnation pressure of the fluid, also known as a total pressure or (particularly in aviation) a pitot pressure. Pitot tubes can become clogged, frozen or otherwise blocked so that air cannot flow to appropriate instruments.